teleconverter for Sigma 70-300mm APO DG macro

Is there a teleconverter that works with Sigma's 70-300mm APO DG macro lens?

Yes.
My Kenko Pro 300DG 1.4 x TC works but you lose auto focus quickly.
It'll become a f5.6-8 lens and won't autofocus consistantly on less than a 1d series body.
If it's a bright day and you tape the end contacts it works ok though

Image degradation may be very substantial as that lens gets soft at 300mm.
I wouldn't recommend it, it may actually be better to crop in photoshop rather than use a TC on that lens.

interesting. I don't mind MF one bit. And strapping a 1.4X TC would really push that 300mm to 420mm. And for a 1.6 cropped cam, it's about 670mm!!! Whoa!

yea, wait till you try a bigma, with a crop, it's more like a 80-800mm zoom.